1. Field of the Invention
This invention relates to a wave attenuation panel that is inserted between a power plant and an air intake of an aircraft nacelle.
An aircraft propulsion system comprises a nacelle in which a power plant that is connected by means of a mast to the rest of the aircraft is arranged in an essentially concentric manner.
2. Description of the Related Art
As illustrated in FIG. 1, the nacelle comprises an air intake 10 at the front, and said air intake makes it possible to channel an air flow into a power plant 12, whereby a first portion of the entering air flow, called primary flow, passes through the power plant to participate in the combustion, and whereby the second portion of the air flow, called secondary flow, is entrained by a fan and flows into an annular pipe that is delimited by the inside wall of the nacelle and the outside wall of the power plant.
The air intake 12 comprises a lip 14 whose surface that is in contact with the aerodynamic flows is extended inside the nacelle via an inside pipe 16 with essentially circular cross-sections and outside the nacelle via an outside wall 18 with essentially circular cross-sections.
The air intake 12 is connected to the power plant by flange-type connecting means 20 comprising, on the one hand, at the end of the inside pipe 16, an annular collar 22 that offers a first support surface and, on the other hand, at the power plant, an annular collar 24 that offers a second support surface that can rest against the first, bolts 26, rivets or the like being distributed over the circumference of the collars 22 and 24 to keep them flattened against one another and thus to ensure the connection between the air intake and the power plant.
Techniques that have been developed to reduce the noise emitted by an aircraft, and in particular the noise emitted by the propulsion systems, consist in placing a coating 28 whose purpose is to absorb a portion of the sound energy, in particular by using the principle of the Helmholtz resonators, at the level of, in particular, the wall of the inside pipe 16. So as to optimize the acoustic treatment, this coating 28 is to extend over the largest surface and generally extends from the collar 22 up to the lip 14.
In a known manner, a coating for the acoustic treatment 28, also called an acoustic attenuation panel, comprises—from the outside to the inside—an acoustically resistive porous layer 30, at least one alveolar structure 32, and a reflective or impermeable layer 34.
The acoustically resistive layer is a porous structure that has a dissipative role, partially transforming into heat the acoustic energy of the sound wave that passes through said layer. It comprises so-called open zones that can allow acoustic waves and other so-called closed or solid waves to pass, not allowing the sound waves to pass but designed to ensure the mechanical strength of said layer. This acoustically resistive layer is characterized in particular by an open surface area ratio that varies essentially as a function of the engine, components that constitute said layer.
According to another constraint, in certain cases of flight, the air flow that enters the nacelle and that has to supply the power plant exhibits a high distortion of pressure and speed that can generate a pumping effect in the power plant. The blades of the power plant are then driven by an oscillating movement along the longitudinal axis of the nacelle that generates a wave that propagates in the inside pipe 16, increasing in intensity until one or more blades breaks.
A first solution to remedy this problem consists in increasing the mechanical strength of the blades so as to limit this oscillating movement. However, this solution is not satisfactory because it leads to increasing the weight of the blades and therefore the on-board weight.
Another solution consists in providing a wave attenuation panel 36 that comprises, like the acoustic attenuation panel from the outside to the inside, a layer that is permeable to certain waves, at least one alveolar structure, and a reflective or impermeable layer. Thus, this panel is intended to absorb in its passage the wave(s) produced by the pumping effect of the power plant and to limit its propagation. According to one embodiment, this panel 36 extends over the circumference of the inside pipe 16, in place of a portion of the acoustic attenuation panel 28.
The characteristics of this wave attenuation panel are determined based on the wave that is to be attenuated.
For this purpose, this wave attenuation panel 36 has a thickness (in the radial direction) that is larger than that of an acoustic attenuation panel 28, on the order of three times greater, and it has a width (in the longitudinal direction) that varies based on the position of said panel in the inside pipe 16. Thus, the closer the wave atteuation panel 36 is to the lip 14, the larger its width, and conversely, the closer it is to the power plant, the smaller its width.
Consequently, there is a tendency to place this wave attenuation panel 36 close to the power plant so as to reduce its dimensions and therefore its weight.
Even if it limits the risks of the blades breaking, the presence of this wave attenuation panel 36 is not without impact.
Thus, this presence tends to reduce the surface area of the coating 28 for the acoustic treatment and therefore to reduce the performance levels of said treatment.
In addition, it is necessary to provide reinforcements 38 on either side of the panel 36 over its entire circumference so as to hold it, which contributes to increasing the on-board weight.